zz * X! W' q" {. J" i, @5 e: B! l# TTypically engine Manufacturers specify 10W-30 lubricants down to around -15F and 5W-30 lubricants to around -22F. These are continuous temps so spasmodically below a little won't be an issue. As others have alluded to though it may "test" the battery and starter a bit!